Majority Leader Harry Reid, D-Nev., said he wanted to bring a gun bill to the full Senate that would have enough support to overcome any GOP attempts to prevent debate from even starting. He expressed concern that including the assault weapons provision might effectively block passage of any bill at all.

Instead, the sponsor of the provision, Sen. Dianne Feinstein of California, said she will offer her ban on the military-style firearms as an amendment. But Feinstein is all but certain to need 60 votes from the 100-member Senate to prevail, and she faces solid Republican opposition as well as likely defections from some Democrats.

“I very much regret it,” Feinstein, D-Calif., told reporters of Reid’s decision. “I tried my best.”

Reid said that “using the most optimistic numbers,” there were less than 40 votes for Feinstein’s ban. That is far less than the 60 needed to begin considering legislation.

“I’m not going to try to put something on the floor that won’t succeed. I want something that will succeed. I think the worst of all worlds would be to bring to something to the floor and it dies there,” Reid said.

Feinstein, an author of the 1994 assault weapons ban that expired after a decade, said that Reid told her of the decision on Monday.

There are 53 Democrats in the Senate, plus two independents who usually vote with them.

An assault-type weapon was used in the December massacre at an elementary school in Newtown, Conn., that revived gun control as a top issue in Washington. Banning those firearms was among the proposals President Barack Obama made in January in response to those slayings.

The assault weapons ban was the most controversial of the major proposals to restrict guns that have been advanced by Obama and Senate Democrats. Because of that, it had been expected that the assault weapons measure would be left out of the initial package the Senate considers, with Democrats hoping the Senate could therefore amass the strongest possible vote for the overall legislation.

Having a separate vote on assault weapons might free moderate Democratic senators facing re-election next year in Republican-leaning states to vote against the assault weapons measure, but then support the remaining overall package of gun curbs.

Gun control supporters consider a strong Senate vote important because the Republican-run House has shown little enthusiasm for most of Obama’s proposals.

Feinstein said Reid told her there will be two votes.

One would be on her assault weapons ban, which also includes a ban on ammunition magazines that carry more than 10 rounds of ammunition. The second would just be on prohibiting the high-capacity magazine clips.

Many Democrats think the ban on large-capacity magazines has a better chance of getting 60 votes than the assault weapons ban.

The Senate Judiciary Committee has approved four gun control measures this month, including Feinstein’s barring assault weapons and high capacity magazines. The others would expand required federal background checks for firearms buyers, increase federal penalties for illegal gun trafficking and boost school safety money.